Astronomer Willem de Sitter calculated the size of the universe and studied dark matter, the satellites of Jupiter, and the rotation of the Earth. His theoretical work was groundbreaking, and his name is still heard in such terms of astrophysics as the de Sitter effect, de Sitter precession, and the n-dimensional de Sitter space. Responding to Albert Einstein's field equations of general relativity, de Sitter proposed that space itself is expanding independently of its matter, and eventually Einstein agreed. De Sitter's concept of expanding space, called a de Sitter universe, formed the basis of the Big Bang Theory.